Treatment of metal surfaces



-ride ions.

United States Patent 3,307,980 TREATMENT OF METAL SURFACES Dennis B. Freeman, Harrow, Middlesex, England, as-

signor to Hooker Chemical Corporation, Niagara Falls, N .Y., a corporation of New York No Drawing. Filed Aug. 13, 1963, Ser. No. 301,906 Claims priority, application Great Britain, Aug. 15, 1962,

8 Claims. (Cl. 1486.21)

The present invention broadly relates to the art of coating aluminum and alloys thereof, and more particularly is directed to an improved solution composition and method for providing adherent corrosion resistant chromate coatings on aluminum and alloys of aluminum consisting predominantly of aluminum.

The formation of chromate coatings on aluminum and aluminum alloy surfaces for providing improved corrosion protection or a base more receptive to an overlying paint coating is in extensive commercial use and has been the subject of extensive research and development. Treating solutions for providing such chromate coatings conventionally comprise an aqueous acidic solution containing hexavalent chromium and either simple or complex fluo- The coatings obtained range from a yellow to brown color and can be further modified by incorporating suitable organic dyes in the solution resulting in a coated article having an enhanced decorative appearance. The fluoride ion in the solution acts as an activator to accelerate the coating reaction. In spite of the inclusion of this activator agent, however, treatment times are relatively long and the coatings produced are frequently of a relatively low adherent quality.

It is, accordingly, a principal object of the present invention to provide an improved chromate treating solution for the treatment of aluminum and alloys of aluminum consisting predominantly of aluminum and which solution composition increases the rate at which a coating is formed on the surfaces of the articles being treated and enhances the adherent characteristics of the coating formed.

Another object of the present invention is to provide an improved process for forming adherent chromate coatings on aluminum and aluminum alloy surfaces and which process effects a more rapid formation of the coating than processes of similar type heretofore known.

Still another object of the present invention is to provide an improved chromating solution and process for forming adherent chromate coatings on aluminum and aluminum alloy surfaces and which solution and process are simple to control, of economical preparation and of eflicient and versatile use.

The foregoing and other objects and advantages of the present invention are achieved by the inclusion of a controlled quantity of an alpha-amino acid in an aqueous solution containing hexavalent chromium and fluoride ions within the quantities conventionally employed and which solution is applied to the surface of the aluminum or aluminum alloy article by brushing, immersion, flooding, spraying, or the like. In accordance with a preferred composition of the solution comprising the present invention, a controlled quantity of a selenium compound is incorporated in the chromating solution providing an increased rate in coating formation and an intensification in the color of the coating produced.

Other objects and advantages of the present invention will become apparent on the reading of the following description and the examples provided illustrating typical satisfactory solution compositions and methods of applying the solutions in accordance with the practice of the present invention.

' The chromating solution, in accordance with the pres- 3,307,980 Patented Mar. 7, 1967 ICC ent invention, comprises an aqueous solution incorporating conventional quantities of hexavalent chromium and fluoride ions to which a controlled quantity of either one or a mixture of suitable alpha-amino acids are added. The chromating treating solution contains chromium (measured as CrO in an amount broadly ranging from about 1 up to about 10 grams per liter and a fluoride content present in a concentration broadly ranging from about 0.05 to about 5 grams per liter. The acidity of the solution is conventionally held within a pH range of between about 1 and about 4.

The increased activity of the solution and improved adherence of the chromate coatings obtained in accordance with the discovery comprising the present invention, is based on the addition of a controlled quantity of an alpha-amino acid to the solution within an amount ranging from about 0.1 to about 10 grams per liter. The amino acid additive can comprise a primary, secondary, or tertiary aliphatic alpha-amino acid in which one or both of the hydrogen atoms of the amino group may be substituted in accordance with the general formula as set forth below:

In accordance with the characteristic formula as set forth above, the amino group is attached to the alpha carbon of the acid molecule. The simplest acid corresponding to this formula is glycine wherein R, R and R are hydrogen atoms. A secondary alpha-amino acid is typified by sarcosine (N-methyl glycine) wherein either R or R" correspond to the methyl group and R corresponds to a hydrogen atom. A more complex alphaamino acid incorporating a tertiary amino group is typi fied by ethylene diamine tetra-acetic acid (EDTA). The alpha-amino acid additive can be introduced into the solution either in the acid form or alternatively as a metal salt of the acid employing a compatible metal which will not interfere with the chromating reaction such as, for example, the alkali metals.

The hexavalent chromium constituent of the treating solution may conveniently be introduced as chromic acid, chromates or dichromates, as well as mixtures thereof while the fluoride ions in the solution may be derived.

from hydrofluoric acid, a simple fluoride such as an alkali metal fluoride salt, a fluosilicate, a fluoborate, for example, as well as mixtures thereof.

-In the treatment of aluminum or alloys of aluminum consisting predominantly of aluminum, the treating solution comprising the present invention effects solution of aluminum from the surfaces of the articles being treated which accumulates in the solution as a complex fluoride ion. The accumulation of dissolved aluminum in the treating solution can readily be controlled in accordance with the method described in United States Patent No. 2,967,791, granted January 10, 1961 and assigned to the same assignee as the present invention. As described in the aforementioned United States patent, a portion of the treating solution is passed through a strongly acidic cat ion exchange resin bed and the effluent therefrom is returned to the processing solution. The cation exchange resin effects a removal of the dissolved aluminum maintaining the aluminum concentration in the operating solution in a range preferably of from about 0.005% to about 0.4% by Weight. When the treating solution comprising the present invention is treated in accordance with the process described in the aforementioned patent, it is usually preferred to introduce the alpha-amino acid in the form of an acid instead of a metallic salt since the introduction 3 of such extraneous metal ions to the solution has a tendency to exhaust the cation exchange resin more quickly.

It is also contemplated within the scope of the present invention that the aqueous acidic treating solution containing hexavalent chromium, the fluoride ion, and the appropriate quantity of the alpha-amino acid additive, may also contain a small quantity of a selenium compound present in an amount ranging from about 0.05 to about 1 gram per liter calculated as SeO The selenium compounds which are suitable for this purpose are soluble compounds such as, for example, selenium dioxide, selenious acid, selenic acid, selenates, selenites, as well as mixtures thereof of which selenium dioxide constitutes the preferred material. The inclusion of the selenium compound in the solution has been found to provide still further improvements in the rate of coating formation on the aluminum surface and also contributes to an intensification of the color of the coating produced.

The aqueous acidic'chromating solution either with or without the selenium additive, can readily be applied to the surfaces of aluminum and aluminum alloy workpieces by any of the conventional techniques known in the art including brushing, immersion, flooding, spraying or the like. Due to the high reactivity of the coating solution comprising the present invention, it has been found that it is now possible to obtain satisfactory coatings by spray application of the treating solution at a temperature ranging from about 70 F. to about 130 F. for time periods as low as about 5 seconds. The high reactivity of the solution and the improved coatings produced thereby makes the solution eminently satisfactory for use in spray processes through which continuous strip material is passed.

In order to further illustrate the improved treating solution and process comprising the present invention, the following examples are provided. It will be understood, however, that the examples are provided for illustrative purposes and are not intended to be limiting of the scope of the present invention as set forth in the subjoined claims.

Example 1 In order to compare the improved coating characteristics and the improved coatings produced in accordance with the solution and process comprising the present invention, comparisons were made of the coating characteristics of a chromating solution devoid of the alpha-amino acid additive in relation to a treating solution containing a controlled quantity of glycine. The conventional chromating solution contained 6 grams per liter CrO and 2 grams per liter H SiF which was applied by spraying to the surface of a commercially pure aluminum panel at a temperature of 120 F. for about '15 seconds. A thin, dusty coating Weighing 20 milligrams per square foot was obtained. The same solution incorporating therein 1.5 grams per liter glycine when applied to a similar panel under the same conditions, produced an adherent goldenyellow coating which was not dusty and which weighed 35 milligrams per square foot. The further addition of 0.2 gram per liter of selenium oxide to the aforementioned chromating solution containing the glycine acid additive, applied under the same conditions, also produced golden yellow coatings but the weight of the coating was further increased to 50 milligrams per square foot.

Example 2 A conventional chromating solution was prepared containing 6 grams per liter potassium dichromate (K Cr O and 10 grams per liter of 40% fluoboric acid (HBF which was sprayed onto an aluminum panel for 1 minute at a temperature of 130 F. A thin, poorly adherent :oating was obtained. The addition of 2 grams per liter of the disodium salt of ethylene diamine tetra-acetic acid :0 the solution produced an adherent golden-yellow coating under the same conditions.

Example 3 Three sets of identical aluminum panels were treated as follows:

Set Adegreased only.

Set Bdegreased and treated for 15 minutes at F. in a solution of proprietory nature believed to contain chromic acid, sodium fiuoborate and potassium ferricyanide.

Set Cdegreased and treated for 15 seconds at 120 F.

in a solution containing 5 grams per liter CrO 4 milliliters per liter HBF (40% solution), 2 milliliters per liter HF (40% solution), 1 gram per liter glycine, and 0.05 gram per liter SeO remaining SetA 4 Set B 86 Set C 97 The foregoing tests substantiate the superiority of the chromate coating produced in accordance with the solution and process comprising the present invention as a base for paint film substantially increasing its adherence to the aluminum coated surface.

The solution comprising the present invention can also be simply replenished during use by adding appropriate proportions of the individual constituents to the operating solution or, alternatively, by the addition of an appropriate amount of an aqueous concentrate of the constituents to maintain the concentration of the ingredients within the ranges as hereinbefore set forth.

While it will be apparent that the preferred embodiments herein illustrated are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. A chromating solution for producing an improved chromate coating on the surfaces of aluminum and aluminum alloys consisting predominantly of aluminum comprising an aqueous acidic mixture containing a hexavalent chromium compound present in an amount of from about 1 to about 10 grams per liter calculated as CrO an activator agent comprising fluoride ions present in an amount of from about 0.05 to about 5 grams per liter, and an aqueous soluble aliphatic alpha-amino acid compound selected from the group consisting of primary, secondary, and tertiary alpha-amino acids, compatible metal salts thereof, as well as mixtures thereof present in an amount from about 0.1 to about 10 grams per liter.

2. A chromating solution for producing an improved chromate coating on the surfaces of aluminum and alloys consisting predominantly of aluminum comprising an aqueous acidic mixture containing a hexavalent chromium compound present in an amount of from about 1 to about 10 grams per liter calculated as CrO an activator agent comprising fluoride ions present in an amount of from about 0.05 to about 5 grams per liter, an aqueous soluble aliphatic alpha-amino acid compound selected from the group consisting of primary, secondary, tertiary alphaamino acids and compatible metal salts thereof as Well as mixtures thereof present in an amount of from about 0.1 to about 10 grams per liter; and an accelerator agent comprising an aqueous soluble non-precipitate forming selenium compound present in an amount from about 0.05 to about 1 gram per liter calculated as SeO i 3. A chromating solution for producing an improved chromate coating on the surfaces of aluminum and aluminum alloys consisting predominantly of aluminum comprising an aqueous acidic mixture having a pH of from about 1 to about 4 and containing a hexavalent chromium compound present in an amount ranging from about 1 to about grams per liter calculated as CrO an activator agent comprising fluoride ions present in an amount from about 0.05 to about 5 grams per liter, and an aqueous soluble aliphatic alpha-amino acid compound selected from the group consisting of glycine, sarcosine, ethylene diamine tetra-acetic acid, the alkali metal salts thereof, as well as mixtures thereof present in an amount of from about 0.1 to about 10 grams per liter.

4. A chromating solution for producing an improved chromate coating'on the surfaces of aluminum and aluminum alloys consisting predominantly of aluminum comprising an aqueous acidic mixture having a pH of from about 1 to about 4 and containing a hexavalent chromium compound in an amount of from about 1 to about 10 grams per liter calculated as CrO an activator agent comprising fluoride ions present in an amount of from about 0.05 to about 5 grams per liter, an aqueous soluble aliphatic alpha-amino acid compound selected from the group consisting of glycine, sarcosine, ethylene diamine tetra-acetic acid, the alkali metal salts thereof, as well as mixtures thereof; and an accelerator agent comprising an aqueous solu-ble nonprecipitate forming selenium compound present in an amount from 0.05 to about 1 gram per liter calculated as SeO 5. A chromating solution for producing an improved chromate coating on the surfaces of aluminum and aluminum alloys consisting predominantly of aluminum comprising an aqueous acidic mixture having a pH'of from about 1 to about 4 and containing a hexavalent chromium compound selected from the group consisting of chromic acid, chromates, dichromates, and mixtures thereof present in an amount ranging from about 1 to about 10 grams per liter calculated as CrO an activator agent comprising fluoride ions introduced by a water soluble fluorine compound selected from a group consisting of hydrofluoric acid, simple fluorides, fluosilicates, fluoborates, and mixtures thereof present in an amount from about 0.05 to about 5 grams per liter, and an aqueous soluble aliphatic alpha-amino acid compound selected from the group consisting of glycine, sarcosine, ethylene diamine tetra-acetic acid, the alkali metal salts thereof, as well as mixtures thereof present in an amount of from about 0.1 to about 10 grams per liter.

6. A chromating solution for producing an improved chromate coating on the surfaces of aluminum and aluminum alloys consisting predominantly of aluminum comprising an aqueous acidic mixture having a pH of from about 1 to about 4 and containing a hexavalent chromium compound selected from the group consisting of chromic acid, chromates, dichromates, and mixtures thereof present in an amount ranging from about 1 to about 10 grams per liter calculated as CrO an activator agent comprising fluoride ions introduced by a water soluble fluorine compound selected from a group consisting of hydrofluoric acid, simple fluorides, fluosilicates, fluoborates, and mix tures thereof present in an amount from about 0.05 to about 5 grams per liter, and an aqueous soluble aliphatic alpha-amino acid compound selected from the group consisting of glycine, sarcosine, ethylene diamine tetra-acetic acid, the alkali metal salts thereof, as well as mixtures thereof present in an amount of from about 0.1 to about 10 grams per liter, and an accelerator agent comprising an aqueous soluble nonprecipitate forming selenium compound selected from the group consisting of selenium di oxide, selenious acid, selenates, selenites, and mixtures thereof present in an amount of from about 0.05 to about 1 gram per liter calculated as SeO 7. The method of producing an improved chromate coating on the surfaces of aluminum and aluminum alloys consisting predominantly of aluminum which comprises the steps of applying to the surface an aqueous acidic soluti-on containing a hexavalent chromium compound present in an amount of from about 1 to about 10 grams per liter calculated as CrO an activator agent comprising fluoride ions present in an amount of from about 0.05 to about 5 grams per liter, and an aqueous soluble aliphatic alphaamino acid compound selected from the group consisting of primary, secondary, and tertiary alpha-amino acids, compatible metal salts thereof, as well as mixtures thereof present in an amount of from about 0.1 to about 10 grams per liter, and continuing the application of said solution until a coating results.

8. The method of producing an improved chromate coating on the surfaces of aluminum and aluminum alloys consisting predominantly of aluminum which comprises the steps of applying to said surfaces an aqueous acidic solution at a temperature ranging from about F. to about F. and containing a hexavalent chromium compound, an activator agent comprising fluoride, an aqueous soluble aliphatic alpha-amino acid compound selected from the group consisting of a primary, secondary, and tertiary alpha-amino acid, compatible metal salts thereof, as well as mixtures thereof present in an amount of from about 0.1 to about 10 grams per liter, and an accelerator agent comprising an aqueous soluble nonprecipitate forming selenium compound present in an amount of from about 0.05 to about 1 gram per liter calculated as 8e0 and continuing the application of said solution until a coating results.

References Cited by the Examiner UNITED STATES PATENTS 2,322,205 6/1943 DeLong 148-62 X 2,438,740 3/1948 Clark et al. 1486.2 X 2,507,956 5/1950 Bruno et a1 148--6.21 X 2,522,474 9/1950 Waitkins et al. 148-6.21 X 2,613,165 10/1952 Fischer 1486.2 2,786,002 3/1957 Ambler et a1 1486.2 2,904,413 9/1959 Hampel 1486.2 X

FOREIGN PATENTS 577,731 5/ 1946 Great Britain.

889,322 2/ 1962 Great Britain.

986,455 3/ 1965 Great Britain.

990,352 4/ 1965 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

RALPH S. KENDALL, Examiner. 

1. A CHROMATING SOLUTION FOR PRODUCING AN IMPROVED CHROMATE COATING ON THE SURFACES OF ALUMINUM AND ALUMI1 TO ABOUT 10 GRAMS PER LITER CALCULATED AS CRO3, AN ACTIVATOR AGENT COMPRISING FLUORIDE IONS PRESENT IN AN AMOUNT OF FROM ABOUT 0.05 TO ABOUT 5 GRAMS PER LITER, AND AN AQUEOUS SOLUBLE ALIPHATIC ALPHA-AMINO ACID COMPOUND SELECTED FROM THE GROUP CONSISTING OF PRIMARY, SECONDARY, AND TERTIARY ALPHA-AMINO ACIDS, COMPATIBLE METAL SALTS THEREOF, AS WELL AS MIXTURES THEREOF PRESENT IN AN AMOUNT FROM ABOUT 0.1 TO ABOUT 10 GRAMS PER LITER. 